It is known to extend or retract a telescopic boom by means of a movement arrangement comprising a hydraulic cylinder and one or more chains.
For example, GB 2080244 discloses a telescopic boom including a base section, an inner mid section, an outer mid section and a fly section. A hydraulic cylinder has a rear end connected to a side wall of the base section and a piston rod fixed relative to a side wall of the inner mid section. A first pair of extension chains have respective first ends anchored to a bottom plate of the base section and respective second ends anchored to a bottom plate of the outer mid section. The extension chains of the first pair are wound around respective sprockets at the rear end of the inner mid section. The telescopic boom disclosed in GB 2080244 further comprises a second pair of extension chains having respective first ends anchored to a bottom plate of the inner mid section and respective second ends anchored to a bottom plate of the fly section. The extension chains of the second pair are wound around respective sprockets fixed to a rearward end of the outer mid section. Retraction chains are further provided in order to retract the telescopic boom.
A drawback of the telescopic boom disclosed in GB 2080244 is that, since both the pairs of extension chains are located on the bottom side of the boom, in operation stresses are generated that are not uniformly distributed.
This may result in larger deflections of the sections of the telescopic boom. Consequently, it is necessary to adopt larger wall thicknesses, enlarged cross-sections, and/or better performing materials.
Further examples of telescopic booms having sections moved by a combination of hydraulic cylinders and chains are disclosed in Chinese utility models no. CN 201980948U and CN 201560104U.